1. Field of the Invention
The present invention relates to a metal-made carrier body, for carrying thereon an exhaust gas cleaning catalyst, which is generally provided as an exhaust gas cleaning means at an intermediate point of an exhaust system of an automobile.
More particularly, the invention relates to a metal-made carrier body for carrying thereon an exhaust gas cleaning catalyst, in which the individual components of the carrier body can perform their function without separation, breakage or other damage when used even in severe environments.
2. Description of the Related Art
Conventional metal-made carrier bodies of the above sort, which are adapted to carry an exhaust gas cleaning catalyst thereon, include those having a structure formed by laminating at least one sheet-like metal band and at least one corrugated metal band in layers or rolling them together into a multi-layered composite body with a view toward increasing the carrying area per unit volume, namely, with a view toward increasing as much as possible the effective area of contact between exhaust gas and the exhaust gas cleaning catalyst per unit volume and further reducing the weight of the metal-made carrier body to a maximum possible extent.
For example, a sheet-like metal band made of a heat-resistant thin sheet of Fe-Cr steel having a thickness of 0.1 mm or smaller and a corrugated band made from another thin metal sheet of the same type are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a multi-layered composite body defining many network-patterned gas flow passages along a central axis thereof for allowing exhaust gas to pass therethrough. The multi-layered composite body is enclosed within a tubular metal casing which has a single-layer structure and opens in opposite ends thereof. The members of the multi-layered composite body, i.e., the sheet-like band and corrugated band are put together into a vibration-proof structure. Namely, the sheet-like band and corrugated band as well as the multi-layered composite body thus rolled and the metal casing are put together at the areas of contact therebetween by welding, brazing or the like. These joining means at the areas of contact between the multi-layered composite body and the metal casing are exemplified by the following:
(i) Electron beam welding, laser beam welding, arc welding, etc. as disclosed in Japanese Patent Laid-Open Publication No. 13462/1979 and Japanese Patent Laid-Open Publication No. 1585/1982, for example. PA1 (ii) Brazing, as disclosed in Japanese Patent Laid-Open Publication No. 4373/1981. This brazing includes applying a paste of binder and brazing material (or applying a powdery brazing material after painting a binder, or applying a sheet of brazing material) over a desired portion of at least one of the two members, i.e. the sheet-like band and the corrugated band, then rolling or laminating these two members together to provide a multi-layered composite body, and finally heating around the above-mentioned portion of the multi-layered composite body. PA1 (iii) Brazing, as disclosed in Japanese Patent Laid-Open Publication No. 199574/1986 (division of Japanese Patent Laid-Open Publication No. 4373/1981). This brazing includes rolling or laminating the sheet-like band and the corrugated band together to provide a multi-layered composite body, then applying a brazing material to the ends of the multi-layered composite body such as by painting or spraying, and finally heating around the ends of the multi-layered composite body. PA1 (iv) Another brazing includes electroplating or electroless-plating to form a coating of brazing material on the surfaces of the sheet-like and corrugated bands before or after the sheet-like and corrugated bands are shaped into a multi-layered composite body, and then heating the coated surfaces.
The above conventional joining means cause the following production and quality problems:
With the joining method of (i) above, spot-welding must take place at narrow areas of contact or interior areas of contact, which is laborious and time-consuming and hence causes only an inadequate degree of vibration-proofness. Further, electron beam welding and laser beam welding require special and expensive equipments. This would necessarily result in a reduced rate of production.
In the joining method of (ii) above, it is necessary to use an expensive brazing material such as nickel, and the way of applying and drying the brazing material is complex and laborious. As the multi-layered composite body, in which the the sheet-like and corrugated bands are rolled together and in which the brazing material is applied only to the restricted areas, is enclosed in a tubular metal casing and is brazed at the restricted areas of contact between the sheet-like and corrugated bands by heating, the multi-layered composite body would vary in size so that the roll becomes loose to create gaps at the areas of contact. This is partly because a part of the brazing-material coating is melted to flow out from the areas of contact between the sheet-like and corrugated bands when heated, and partly because the sheet-like and corrugated bands are rolled in layers to accumulate the loosed amount. Thus adequate vibration-proofness is difficult to achieve. Further, during the heat-treatment as well as when the metal-made carrier body of this type is installed in the exhaust system of an automobile, it is highly likely that gaps be formed due to the high temperature of the exhaust gas with the lapse of time.
Also with the joining method of (iv) above, it is impossible to join the sheet-like and corrugated bands with adequate firmness. Therefore adequate vibration-proofness cannot be achieved.
In the joining methods of (iii) and (iv) above, it is impossible to join the sheet-like and corrugated bands especially centrally of the metal-made carrier body, and the application of the brazing material and the plating require laborious work, as described above in connection with (ii) above. Further, because an expensive vacuum furnace is used, the joining method of each of (ii), (iii) and (iv) is not economical with a view to their running costs.
In the above-mentioned conventional production of the metal-made carrier body for an exhaust gas cleaning catalyst, there was a limit in preventing the constituent members of the carrier body from any separation.
Further, the metal-made carrier body of this type is used in severe thermal environments and hence requires adequate durability against any breakage, cracking or separation of the members making up the the carrier body. In the exhaust system of an automobile, the members of the carrier body are subjected to strains (hereinafter called "thermal strains") due to the large difference between their coefficients of thermal expansion (volume expansion and linear expansion) under the severe cooling and heating cycle of from -20.degree. C. to 900.degree. C. These members are also subjected to vibrations from the automobile body. As a result, the members would be separated, cracked or otherwise damaged.